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Solar Guide

How to Size a Battery Bank

Learn how to size a solar battery bank for backup power, off-grid homes, and daily energy storage.

6 min read

Introduction

A solar battery bank stores energy so your system can power loads at night, during cloudy weather, or when grid power is unavailable. Good battery sizing starts with daily energy demand and then accounts for autonomy, usable capacity, battery type, and system voltage.

Key Concepts

  • Daily load determines the base amount of energy the battery must supply.
  • Autonomy days define how long the system should operate without strong solar input.
  • Depth of discharge limits how much stored energy should be used to protect battery life.
  • Battery efficiency and inverter efficiency reduce the usable energy available to appliances.

Practical Examples

  • A 2,000 Wh/day load with 2 autonomy days requires 4,000 Wh before efficiency and depth-of-discharge adjustments.
  • For lithium batteries with 80% usable depth of discharge, that same system may need about 5,000 Wh or more of nominal battery capacity.
  • At 24 V, a 5,000 Wh battery bank is about 208 Ah before final design margin and manufacturer-specific limits.

Common Mistakes

  • Using total battery capacity instead of usable capacity.
  • Forgetting inverter losses between batteries and AC loads.
  • Undersizing batteries for pumps, refrigerators, or nighttime loads.
  • Mixing battery types, ages, or capacities without proper engineering review.

Try Solar Home Planner

Use Solar Home Planner to estimate battery amp-hours and storage capacity from real daily loads.

Try Solar Home Planner